1. Field of the Invention
The invention relates to material deposition and, in particular, to material vapor deposition.
2. Art Background
Many processes have been developed for the deposition of materials, e.g., semiconductor materials, on a substrate. On such process involves the use of a precursor gas, i.e., a gas that upon contact with the substrate undergoes a modification such as a chemical reaction to yield a deposited layer. (Typically, the precursor gas is a mixture of gaseous components.) In these vapor deposition processes, generally, the gas flow and its spatial relationship to the substrate are carefully controlled. For example, in the most common spatial configuration employed in chemical vapor deposition (CVD), a gas flow is established at one end of a vessel, a substrate is placed within the vessel, as shown in FIG. 1, and a gas flow is established in the direction of arrows, 10, parallel to the major surface of the substrate, 12. In an alternative configuration employed in CVD processes, the substrate is positioned as shown by phantom substrate, 14, so that the flow direction is generally perpendicular to the major surface of the substrate. The first configuration, i.e., the parallel configuration, is most commonly used because it introduces the least perturbation in the precursor gas flow. However, the latter configuration is at times employed when it is desired to minimize the temperature gradient across the substrate introduced by the corresponding axial temperature gradient in the reactor. In one spatial variation, the substrate is canted to a position between parallel and perpendicular in an attempt to combine the advantages of each configuration.
In another variation, generally denominated close space deposition, a sublimable material is placed at the bottom of a vessel, such as shown in FIG. 2, with the vessel dimensions chosen so that they are essentially coextensive with the dimensions of the substrate, 15. The substrate is held above this vessel, 17, and a vapor is produced by heating the material, 18, and thus inducing sublimation. The resulting vapor diffuses through the vessel and produces film deposition on a substrate maintained at a temperature below that of the subliming material. Close space deposition is typically employed when apparatus simplicity is desired, but it often leads to control difficulties, e.g., thickness and compositional irregularities.
Other configurations are also utilized for specific applications, such as those requiring deposition of a plurality of compositionally dissimilar layers. For example, the configuration shown in FIG. 3 has also been employed. (See "Vapor Phase Epitaxy of III-V Compound Optoelectronic Devices," by G. H. Olsen in Proceedings on the Symposium on III-V Opto-electronics Epitaxy and Device Related Processes, edited by V. G. Keramidas and S. Mahajan, Vol. 83-13, Electrochemical Society, pages 231-251 (1983) for a detailed description.) Basically, the substrate, 20, is positioned at the orifice of a tube, 22, so that its major surface is perpendicular to the long axis of the tube. The precursor gas flow, 25, is then directed along the tube, emerges from the tube, and contacts the substrate. If two such tubes are employed, then it is possible to establish different precursor gas flows through each tube. By a translational shift such as an eccentric rotation around an external shaft as shown at 26, the substrate is first subjected to one gas flow and then to the second at 27. In this manner, deposited layers having different compositions are sequentially formed on a substrate. In one modification, the substrate is actually inserted into the tube in a parallel, perpendicular, or intermediary configuration, and when a composition change is desired, the substrate is withdrawn, rotated eccentrically, and inserted in the second tube. These dual tube techniques produce transitional regions between layers with a compositional gradient that is less severe than that obtained by changing the precursor gas in the previously discussed single gas flow methods. However, in any of the multiple tube techniques, translation of the substrate induces substantial gas flow perturbation. These perturbations induce contamination of one gas flow by the other, and produce a generally undesirable transitional region rather than producing a relatively abrupt compositional change between layers.
Each deposition configuration has been designed to achieve specific objectives and each has been used for specific applications. However, it is desirable to improve layer uniformity and to reduce transitional regions between layers. It is also certainly desirable to enhance the flexibility of processes to achieve the combined attributes of a variety of existing techniques.